1. Field
This disclosure relates to video compression, more particularly to methods of rate control in video compression.
2. Background
Digitization of video sequences allow for easy transmission of these sequences across data networks. Additionally, digitization allows for manipulation of the data to correct artifacts and faults in the sequence. However, a digital representation of a video sequence uncompressed requires too much bandwidth to make transmission very practical. Therefore, most of these sequences are compressed.
The goal of most compression techniques is to provide the maximum amount of compression with least amount of error in the reconstructed technique. Compression generally involves reducing redundant data in such a manner that the redundant data can be recreated at the other end, allowing an accurate reconstruction of the video images. Generally, most techniques attempt to remove both spatially and temporally redundant data. One particular technique is that of predictive coding such as that used in the H.26X family of compression standards from the International Telecommunication Union (ITU), or the MPEG (Moving Pictures Experts Group) family of standards.
Generally, predictive coding techniques rely upon three different types of pictures within their schemes. A picture is analogous to a video frame or field, depending upon the standard. I pictures are intracoded pictures, coded without reference to other pictures. Moderate compression is achieved by reducing spatial redundancy, but not temporal redundancy. They can be used periodically to provide access points in the bit stream where decoding can begin. P frames or pictures are predictive pictures and can use the previous I- or P-picture for motion compensation and may be used as a reference for further prediction. P-pictures offer increased compression compared to I-pictures. B pictures are bidirectionally-predictive pictures and can use the previous and next I- or P-pictures for motion-compensation, and offer the highest degree of compression.
In addition to compression percentage and the error rate in the reconstructed picture, another consideration lies in the area of rate control. Constant bit rate (CBR) control generally desires to provide a constant bit rate of compressed data, avoiding huge swings in bandwidth requirements. The challenge becomes to produce accurate reconstructed images, while providing high compression at a predetermined bit rate.
The MPEG standards have achieved some accomplishments in this area. These standards have become wide spread and most work in this area attempts to maintain compatibility with these approaches. In general rate control processes, such as MPEG-2 TM5 (test model 5) and MPEG-4 Q2, the target bit rate is computed based on the bits available and the last encoded frame bits. If the last frame is complex and uses excessive bits, more bits should be assigned to this frame. However, if there are fewer bits left for encoding, fewer bits will be assigned to this frame because of the required bit rate. This can lead to inaccurate reconstructed images, reducing the image quality and proving the compression technique to be unsatisfactory.
It would be useful to have a bit rate control technique that can operate within established standards, have lower computational complexity and still provide higher quality images.